Now to that white knuckle moment at 34,000 feet. The pilot of a commercial jet suffers a heart attack at the controls. The co-pilot takes action. Two passengers rush to help. Tonight we're learning about heroism inside that plane. And we have the drama moment by moment. Reporter: Chaos in the cockpit, just before 8:00 p.M. On united airline 737 speeding it's way from houston to seattle. The co-pilot radios the tower. We got a man down, chest compressions going on right now. Can an ambulance and maybe some air stairs meet us on the runway? United 1603, we'll get that going. Reporter: The captain is having a massive heart attack over the pa the call goes out, is there a doctor on board? Told everyone to stay seated. Somebody could say they saw them giving the pilot cpr. Reporter: A pair of young army doctors were on board and rushed forward to help. The pilot described the heavy man had to be pulled from the cockpit. The first class cabben turned triage, they performed cpr right there on the floor. I saw them working on the captain. It was very impressive, how they worked together. Reporter: Another plane's pilot heard the commotion and radioed in. What was going on with united? A medical emergency, their captain. That's not good. Reporter: At the same time a united pilot sitting among the passengers went to the cockpit to help as the plane made an emergency landing in boise, idaho. One of the reasons we have two crew members, the airplane can be flown by either of the pilots, each pilot is fully capable of flying the airplane. Reporter: Despite everything their captain didn't make it. He was pronounced dead just a few hours later. Passengers feeling lucky to have landed safely, were eventually put on another plane. Neil karlinsky, abc news, seattle.

This transcript has been automatically generated and may not be 100% accurate.

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